Legalizing drugs would also open up lines of research that are currently not allowed or not useful.
One would be drug interactions. Right now I would not try marijuana, even in a legal state, because I have no idea how it would interact with my diabetes and cholesterol meds. I know to be careful how I ingest alcohol (interacts with glipizide and metformin to drop blood sugar, sometimes to the point of hypoglycemia) and grapefruit (can cause liver damage in interaction with simvastatin) because alcohol and grapefruit are legal. Labs can’t test marijuana interactions because it’s illegal at a national level.
Next, how about tests to measure impacts of drugs on driving ability — breathalyzers or equivalents for marijuana, cocaine, and so on. There’s no market right now, because any amount of an illegal drug is punishable, so the cops don’t care if someone had a puff/snort or is high out of their mind.